Large containers, for example, in the 10 to 20 liter (2.5 to 5.0 gallon) size, have been produced in the past where the lids screw onto the containers. Such lids have been produced by injection molding them in molds that have an annular thread-forming ring. When the lid has been molded and the mold opened, the lid is removed from the mold by rotating the annular ring, effectively to unscrew the mold from the lid. A difficulty with this method of molding these lids, however, is that the thread-forming annular ring has to be sealed in the mold during the molding process. In the past, this has been done by using large diameter O-rings. The problem is that such O-rings deteriorate or wear out very quickly due to the rotation of the annular ring against the O-rings.
The present invention overcomes these problems by eliminating these large O-rings. The thread-forming annular ring is made stationary in the mold and a rotatable mold center section spins the lid after it is molded to unscrew the lid from the mold.